1. Field of the Invention
The present invention relates to long-term underground water recharge, and in particular, to the use of storm and other run-off water, or water that has been collected, to cause such recharge and produce additional benefits therefrom.
2. Problems in the Art
Soil functions as a plant growth medium, a regulator and partitioner of water flow, and as a buffer of environmental change. A National Research Council report reviews soil""s role in the hydrologic cycle (National Research Council. 1993. Soil and Water Quality: An Agenda for Agriculture. National Academy Press. Washington, D.C.). Rainfall in terrestrial ecosystems falls on the soil surface where it either infiltrates the soil or moves across the soil surface into streams or lakes. The condition of the soil surface determines whether rainfall infiltrates or runs off. If it infiltrates the soil, it may be stored and later taken up by plants, move into ground waters, or move laterally through the earth, appearing later in springs or seeps. This partitioning of rainfall between infiltration and run-off determines whether a storm results in a replenishing rain or a damaging flood. The movement of water through soils to streams, lakes, and ground water is an essential component of recharge and base flow in the hydrological cycle (Stephens, D.B. 1995. Vadose Zone Hydrology . Lewis Publications, Boca Raton, Fla. 347 pp.).
Current engineering practices in the Southwest United States and other places often reduce infiltration and increase run-off by collecting and conveying storm water quickly and efficiently for ultimate discharge to an existing surface water course. The goals of this management strategy are to reduce hazards to the public and minimize property damage. Although this strategy maximizes the objectives in terms of safety and risk reduction, it prevents the beneficial use of storm waters through recharge. Conveyance times are minimized while flow rates are maximized to remove it from highways, streets and public areas. With concern growing over the availability of water supplies for public and industrial use, alternative uses of storm water must be considered, and if practical, implemented.
Root systems can increase soil saturated hydraulic conductivity over an order of magnitude (Prieksat, M. A., T. C. Kaspar, and M. D. Ankeny. 1994. Positional and temporal changes in ponded infiltration in a corn field. Soil Sci. Soc. Am. J. 58:181 -184). Increased root biomass results in more root channels as well as an associated increase in faunal activity creating wormholes and other channels. These preferential flow paths increase saturated hydraulic conductivity. In arid and semi-arid systems, plant growth is usually limited by water availability. Because plants maintain fairly constant root:shoot ratios, root mass and soil hydraulic conductivity are often directly correlated with water availability.
Therefore, current practices might produce beneficial results in one sense, namely the more efficient removal of run-off water, and in another sense it can be detrimental.
Most previous buffer zone and riparian zone research (primarily from the Northeast and the Midwest) has operated on the premise that vegetation is necessary to establish control to run-off. In the western United States (U.S.), establishment of vegetation is often problematic due to intermittent water supply. Thus, contrary to what is typically found in the eastern U.S., run-off must be controlled to (re)establish vegetation in arid and semi-arid areas. Run-off control can provide water for establishment of vegetation. One problem is obvious: to obtain the benefits of vegetated buffer zones in arid and semi-arid areas, we need effective methods to convert transient run-off into a steady water source for establishment of vegetation.
Healthy riparian areas provide numerous benefits when viewed as a component of the basic hydrologic unit, i.e., the watershed. Woody riparian species provide channel and bank stability and thus prevent incisement of the channel. Local ground-water levels are maintained due to slow release of bank storage. Natural fluvial processes create channels which efficiently transport water. Water quality is improved where sources of sediment from destabilized banks are eliminated. Shading reduces extreme fluctuations in temperature and evaporative losses from perennial streams. Additionally, riparian zones can attenuate high flood flows while promoting sediment deposition and ground water recharge. These attributes optimize the hydrologic response of a watershed with regards to the storage and discharge of water.
From a soil science perspective, grazing and cropping practices have often reduced soil water storage capacity and increased run-off. A common result has been lowering of local water tables with permanent stream reaches becoming intermittent. From a hydrological perspective (e.g., Menzel, B.W. 1983. Agricultural management practices and the integrity of instream biological habitat. pp. 305-329, in Agricultural Management and Water Quality, F. W. Schaller and G.W. Bailey, eds. Iowa State University Press. Ames, Iowa), these same agricultural practices lead to an exaggerated seasonal flow regime and increase the frequency, severity, and unpredictability of high-volume flows.
Subsurface flow, including that from recharge galleries, generally provides a more constant source of water than surface flow. Obligate phreatophytes, such as cottonwoods, require a constant source of water for survival. Facultative phreatophytes, such as salt cedar (Tamarix sp.), tolerate drier periods. In southwestern riparian zone forests (bosques), replacement of native vegetation (such as willow and cottonwood) by exotics (such as salt cedar and Russian olive) has occurred with watershed degradation over time. The recharge component of the water balance considered critical for maintaining base flow in streams is therefore important in maintaining desired vegetation.
Surface water have been diverted and infiltrated for thousands of years in various parts of the world (Bruins, J. J., M. Evenari, and U. Nessler. 1986. Rainwater-harvesting agriculture for food production in arid zones: the challenge of the African famine. Appl. Geography. 6:13-32). Wills (1988) summarizes much of the literature on prehistoric southwestern U.S. agriculture and water harvesting (Wills, W. H. 1988. Early prehistoric agriculture in the American southwest. 188 pp. School of American Research Press, Santa Fe, N.Mex.). The primary objective of water harvesting is short-term water storage in the soil profile for crop growth. In the southwest, Native American farmers place fields to optimize water and sediment trapping These relationships among soil, vegetation, and erosion/deposition are discussed by Jenny (Jenny, H. 1980. The Soil Resource. Springer-Verlag, New York). Soils derived from aeolian deposits, alluvium and colluvium are generally considered unsuitable for agriculture because of their high infiltration rates (Tabor, J. A. 1995. Improving crop yields in the Sahel by means of water-harvesting. J. of Arid Environments. 30:83-106).
While water erosion processes are complex, as evidenced by an abundant and growing amount of technical literature, the principals of erosion control are often stated as these relatively simple principles. Plant, plant residue, or mulch cover should be increased in intensity or in time to decrease energy and volume of run-off water. In the west, local increase of recharge also may facilitate erosion control. The rationale is this: Increased recharge results in increased bank vegetation, and increased bank vegetation reduces bank erosion. Bank erosion is a major contributor to the sediment load in many western streams.
Therefore, the potential benefit can be seen in the encouragement of vegetation growth, not only for reducing possible erosion, and not only for the increased ability for long-term recharge of underground infiltration galleries, but vegetation can actually increase the holding potential of water underneath the ground and beneficially improve hydrological characteristics of the soil.
Additionally, there is a significant amount of attention on water treatment of run-off water, including by the Environmental Protection Agency (EPA). A substantial number and amount of pollutants can exist in run-off, particularly the first stages of run-off. In the southwest, for example, such things as grease, oil, gasoline, dog waste, and even viruses, can accumulate above ground during dry times. Initial run-off would produce water that is full of such things. If allowed its normal course, the run-off, as explained above, would be collected in storm sewers. It many times would be transported and released in a manner that would allow it to reach ground water, where it could pollute the same. A real need exists for a way to treat such run-off, especially the first run-off, before it reaches ground water.
Still further, there could be advantages to be able to take water which has been previously been collected, for example in a reservoir, and move such water to another location. Water from a reservoir could be used to charge areas downstream of the reservoir.
The primary methods for artificial recharge include: (1) water spreading, (2) infiltration basins, (3) injection wells, and (4) infiltration galleries. Each of these is described briefly below, along with their advantages and disadvantages.
Water spreading simply spreads water upon an existing or graded landform. Water spreading is often used on river beds and has been used in rangeland management. Infrastructure and maintenance costs are low. Because water is infiltrating under approximately unit gradient conditions, infiltration on an areal basis is often low which, in turn, leads to large shallow infiltration areas prone to high evaporation and reduced water quality. Playas constitute a natural example of this hydrological behavior.
Infiltration basins are ponds constructed to maximize the infiltration of water into the underlying soil. Often, infiltration is impeded because the basin bottom tends to clog rapidly with fine sediments, creating a low-permeability (clogging) layer that severely impedes downward flow of water. By allowing the pond to dry out completely between periods of infiltration, higher infiltration rates are achieved. However, intermittent cleaning of the pond bottom is usually required. Graded fill material may be used in the pond bottom to create a filter for fine sediment.
Injection wells are similar to ground-water extraction wells, except that water flows into the well instead of out. In contrast to ponds, galleries, or dry wells, there is no opportunity for removal of suspended solids or dissolved solutes within the vadose (unsaturated) zone because injection wells recharge the aquifer directly within the saturated zone. Therefore, injection wells must use water of higher quality than for the other methods.
Because of the relatively small surface area of a wellbore, as compared for example with infiltration galleries, gradual clogging of injection wells is to be expected without expensive particulate removal.
Clogging is caused by both corrosion of well screens and plugging of the borehole wall by fine sediment. For this reason, regular redevelopment of injection wells is often necessary and accounts for much of the operating expense for this type of system.
Advantages of injection wells include: (1) large infiltration capacity, (2) availability of land surface for other beneficial uses. Disadvantages include: (1) high initial construction costs, (2) potentially high permitting costs, (3) high maintenance costs for well re-development, (4) requirement that feed water be of high quality to avoid aquifer contamination.
Infiltration galleries consist of a network of subsurface pipes set in gravel-filled trenches. The size of the trenches and piping system are dependent on the rate at which water is to be introduced into the subsurface. The permeability of the soil must first be tested using field and/or laboratory methods. Once this is known, mathematical equations exist to determine the trench size and geometry to guarantee adequate infiltration capacity for the planned flow rate. Over design of the infiltration galleries is standard procedure to allow for some clogging of the trench walls over time. Infiltration galleries for artificial recharge have received little attention in the western US.
Where relatively coarse soils are present at the ground surface, infiltration galleries can be among the most efficient means of artificial recharge. Advantages of infiltration galleries include: (1) large infiltration capacity, (2) large subsurface storage volume, (3) longer vadose zone residence time for improved water quality, (4) availability of land surface for other beneficial uses (e.g. forage), and (5) possible conjunctive use as a subsurface irrigation system for plant cover. The main potential disadvantage in this setting is the potential for clogging when using sediment laden waters.
It can therefore be seen that there is a real need in the art, especially in more arid regions, for a system to beneficially utilize or handle storm and other run-off water, in particular, for recharging and storing water underneath the ground, or treating the same. It is therefore a principal object of the present invention to provide an apparatus and method for the beneficial use or handling of run-off water. Other objects, features, and advantages of the invention include an apparatus and method for beneficial use or handling of run-off water which:
1. Is passive.
2. Is low maintenance.
3. Is low cost.
4. Enhances long-term water recharge capabilities of underground infiltration galleries.
5. Encourages vegetation growth and root development.
6. Improves soil conductivity and hydrological characteristics of soil.
7. Reduces or controls erosion.
8. Is durable.
9. Can be used to at least partially treat water.
10. Decreases energy and volume of run-off water.
11. Can be self cleaning.
12. Decreases sediment load in streams.
Another object is to assist in moving previously stored water to recharge underground infiltration galleries. Desirable effects are achieved by cycling xe2x80x98dirtyxe2x80x99 water in an infiltration gallery: i.e., backflushing, maintenance of high permeabilities, and sediment removal.
These and other objects, features, and advantages of the present invention will be become more apparent with reference to the accompanying specification and claims.
An apparatus and method is presented for the beneficial use or handling of run-off or previously collected water. The apparatus includes a siphon or other draining device in combination with a permeable underground water reservoir. A source of run-off or previously collected water is in fluid communication with the permeable water reservoir. The run-off or collected water fills up the permeable water reservoir and infiltrates the ground around the reservoir until it reaches the level that equals the critical head of the siphon or draining device. At that time, the siphon or draining device drains water from the reservoir to an outlet. The outlet essentially takes overflow water to a desired location.
The method directs run-off or previously collected water to an intentionally permeable underground water reservoir. Inflow of run-off or collected water is allowed to go into the reservoir and further infiltrate surrounding soil for recharge of the soil with water or treatment of the water. Inflow continues to cause infiltration of the surrounding soil until inflow causes the level of the reservoir to rise above a critical head. At that time, water is removed from the reservoir. As long as inflow generally equals outflow, an equilibrium will exist. When inflow plus infiltration is less than outflow, the reservoir will be drained down until a predetermined point. At that time, the system is ready for additional inflow and infiltration.